Potentiometer worm screw retention

ABSTRACT

Simple inexpensive means for preventing undesired axial translation of the worm screw and attendant operational changes in a worm screw adjusted resistor irrespective of loose mechanical dimensional tolerances, consisting of providing on the screw a beveled annular conical cam surface and a complementary beveled surface on the interior of the resistor housing and a seat for the interior end of the worm screw, the beveled surfaces coacting to force the end of the screw against the seat irrespective of dimensional tolerance variations.

United States Patent [72] Inventor Robert D. Michik Walnut, Calif. [21]Appl. No. 869,575 [22] Filed Oct. 27, 1969 [45] Patented Apr. 27, 1971[73] Assignee Bourns, Inc.

[54] POTENTIOMETER WORM SCREW RETENTION 6 Claims, 8 Drawing Figs.

[52] US. Cl 338/162 [51] H0lc 9/00 [50] Field of Search 338/(Digest 1),180, 181, 162l75 [56] References Cited UNITED STATES PATENTS 3,399,3678/1968 Tumbusch 338/175 3,522,573 8/l970 Michik 338/171 PrimaryExaminer-Laramie E. Askin Assistant Examiner-D. A. Tone Attorney-FritzB. Peterson ABSTRACT: Simple inexpensive means for preventing undesiredaxial translation of the worm screw and attendant operational changes ina worm screw adjusted resistor irrespective of loose mechanicaldimensional tolerances,- consisting of providing on the screw a beveledannular conical cam surface and a complementary beveled surface on theinterior of the resistor housing and a seat for the interior end of theworm screw, the beveled surfaces coacting to force the end of the screwagainst the seat irrespective of dimensional tolerance variations.

POTEN'I'IOMETER WORM SCREW RETENTION BRIEF SUMMARY OF THE INVENTION a.Background of the Invention In potentiometers and like variableresistors in which a rotatable wiper or contact is traversed along theextent of an arcuate resistance element by rotation of a worm wheel towhich the contact is connected, and which worm wheel is adjustedbyrotation of a worm screw, difficulty has been experienced in providingsimple and inexpensive but effective means for retaining the worm screwin operating position and preventing undesired axial translation of theworm screw. Any such axial movement or end play permits someuncontrolled rotation of the worm wheel and contact with consequentundesired and uncontrolled change of the resistance setting of theresistor.

Many sorts of devices and arrangements have been tried and used inefforts to retain the worm screw in place and prevent such axialtranslation. For example, as shown in U.S. Pat. No. 3,399,367, a keeperblade 65 engages in a circumferential groove in the screw and isrestrained against some movements. As shown in U.S. Pat. No. 3,208,024,a pin 44 engages in a similar circumferential groove for the same pur--'pose. In U.S. Pat. No. 3,150,343, a retainer screw 42 has a reduced endwhich engages in a circumferential groove in the screw. In US. Pat. No.3,108,245, an annular seal 48 and a sleeve 49 are similarly utilized. InU.S. Pat. No. 2,887,554, an annular flange on the worm screw engages insemiannular mating recesses formed in half shell members of the resistorhousing. In each and every one of the noted types of constructions,looseness and axial translation of the worm screw can occur when themanufactured interfitting parts are within allowed dimensional tolerancelimits but one is at or near a positive tolerance limit and the other isat or near a negative tolerance limit. All manufactured parts which arerequired to fit other parts are made with dimensional variations, sinceit is physically impossible to manufacture parts to exact dimensions.Thus in the case of a pin or a key in an annular groove as above noted,if the groove is of maximum permitted width and' the pin of minimumdiameter or width, end play of the shaft or screw is permitted. Only inthe very exceptional case wherein both parts are exactly correctdimensionally, or are both at maximum, or both at minimum, tolerancelimits, will end play or axial translation be prevented.

An additional undesirable feature associated with most if not all of theprior-art solutions to the problem of eliminating possible end play ofthe wormshaft is that of the cost or ex pense involved in (l) producingthe pin, key or like part, and (2) assembling the parts.

b. The Present Invention, Brief Summary The present invention eliminatesthe part exemplified by the pin, key, ring or similar retainer deviceemployed in prior-an variable resistors, and eliminates the laborinvolved in assembling such part, in addition to providing means whichpositively avoids entirely any possibility of undesired worm screw endpl'ay despite liberal permitted dimensional tolerance limits, byproviding a steeply tapered or annular bevel cam surface or portion ofthe shaft, a cooperating arcuate complementarily tapered or beveled camsurface fixed in the resistor housing, a worm wheel having at least aslightly flexible or resilient rim portion, and a thrust seat or bearingon an inner wall of the housing against which the inner end of the wormscrew is forcibly urged by the cam action of the coactin'g bevel camsurfaces of the housing and worm screw. Dimensional variations areaccommodated by the bevel surface on the worm screw traveling along thefixed cam surface in the housing only to the extent permitted byengagement of the end of the worm screw with the seat or bearing, theworm screw assuming the required one of a plurality of possibletransverse-of-axis positions and pressing against the worm wheel withmore or less than a median value of force. Such variation of lateralposi-' tioning of the axis of the worm screw is permitted, without anypossibility f backlash, by the resilience of the worm wheel and theresilience of the material of which the housing body is formed. Only therim portion of the wonn wheel need be of yielding character, but theentire wheel may be of stiff elastic material, such as, for example,nylon.

The preceding brief summary description of the present invention makesit evident that it is a broad general object of the invention to provideimprovements in worm screw adjusted resistors. Another and more specificobject is to provide inexpensive means for retaining in place andpreventing undesirable axial translation of the worm screw in a resistorof the noted type. Another object is to improve the operatingcharacteristics of a worm screw adjusted resistor while making thestructure and assembly thereof less expensive. Other objects andadvantages of the invention are hereinafter set out or made evident inthe appended claims and the following detailed description of apresently preferred exemplary physical embodiment of the inventionillustrated in the accompanying drawings.

DESCRIPTION OF THE DRAWINGS In the drawings;

FIG. 1 is a pictorial view of an exemplary worm screw adjusted variableresistor of the potentiometer class incorporating the invention, thedrawing being to no specific scale;

FIG. 2 is a plan view of the resistor of FIG. 1, with portions brokenaway to reveal internal details, the drawing being to a different scale;

FIG. 3 is a pictorial view of the body of the housing of the resistor ofFIG. 1, to a different scale;

FIG. 4 is a plan view of the resistor housing body, at no particularscale;

FIG. 5 is a sectional view taken as indicated by broken line anddirectors 5-5 in FIG. 4;

FIGS. 6 and 7 are fragmentary sectional views taken as indicated byindicators 6-6 and 7-7, respectively, in FIG. 5; and

FIG. 8 is an enlarged plan view of the worm screw shown in place in FIG.2.

DETAILED DESCRIPTION As used herein, the term resistor is intended tomean a variable resistor of the type commonly referred to as apotentiometer. 1 I

Referring first to FIGS. 1 and 2, the potentiometer indicated generallyby number 10 comprises a boxlike housing composed principally of a body12 preferably of molded or cast insulation and a cover 14 which is aceramic plate that fits in a recess in the body and is held against exitby cement or a sealant potting compound 16. Terminal wires or leads L1,L2 and L3 extend from resistive and conductive films or coats (notshown) on the interior face of the cover to the exterior of the housingthrough a mass 16' of the potting compound disposed in a depression orrecess formed in the body, whereby conventional electrical connectionsare provided for the potentiometer. As indicated in FIGS. 3 and 4, acircular gudgeon or bearing 12a upstanding from the floor of the body isprovided to journal a worm wheel 18 (FIG. 2) which comprises resilientmeans so as to be capable of yielding radially under the influence ofappliedforce. For example, the worm wheel may have a resilient rimportion, or may be formed of resilient synthetic polymer. The worm wheelcarries a conventional contact (not shown) which connects the notedresistive and conductive films in known manner.

The body is preferably of open-top boxlike form, and is provided with abore 12b (FIG. 3) for reception of a worm screw 20 (FIG. 8) that isthreaded for driving engagement with the worm wheel 18. Further thehousing is provided with an elongate abutment which provides an elongatearcuate seat 12d which partially encircles the worm screw and limitsupward and lateral outward movement of the threaded portion 20: of theworm screw. The upper portion of the abutment 12c serves to restrain theworm screw against upward movement when the latter is in operatingrelationship with the worm wheel; and the upper surface of the abutmentserves, in conjunction with a generally arcuate shelf 12f formed in therecess of the body 12, to position and support the active face of theceramic cover 14.

Adjacent a lower corner of the recess formed in body 12 of thepotentiometer housing there is provided a shaped abutment orprotuberance 12e which presents first and second fixed bevel camsurfaces 12m and l2n, respectively. As indicated in FIGS. 3, 4 and 7,the abutment 12s is formed with a radius, that is, it is curved; andhence the stationary bevel cam surfaces 12m and l2n are curved. As isindicated in FIG. 7, the fixed cam surfaces are disposed laterally ofthe axis A of the bore 12b. The latter bore is coaxial with the arcuatesurface 12d of the elongate abutment l2c. Further, the interior wall ofbody 12 of the housing is formed to provide a flat face or bearing 12::against which the circular inner end face 20x of the worm screw is urgedin a manner presently described.

During assembly of the potentiometer, the worm screw is inserted intobore 12b and pressed inward until the endmost annular bevel cam surface20a (FIG. 8) thereon engages the cam surface 12m of the abutment ll2e inthe housing body. Continued inward movement of the worm screw underapplied force results in the inner head 20h of the worm screw beingdeflected laterally by the interaction of the noted cam surfaces,followed by the outer annular'periphery of head 20h snapping past theridge of abutment 12e and sliding movement of cam surface 20b of theworm screw downwardly along the slope of cam surface l2n of theabutment. Thereafter the worm wheel 18 is pressed onto bearing 12a onthe floor of the housing body, with the contact (not shown) extendingupwardly. As the worm wheel is pressed in place it resiliently pressesagainst the worm screw, forcing the latter toward seat 12d and causingcam surface 12n to act on cam surface 20b of the worm screw inner headand thereby move or translate the worm screw axially until bearingsurface 20.x at the inner end of the worm screw engages bearing surface12x on the wall of the body. Thereafter the ceramic cover plate 14,bearing on its inner or lower face an arcuate resistance element andreturn conductor, is pressed into the recess in the body, and cementedor potted in place with the periphery of the lower face resting on shelf12f and upper faces of abutments 12c and 12a of body 12. As the ceramiccover is pressed into position, the resistive and conductive returncoatings thereon are brought into conductive contact with the rotatablecontact carried or driven by the worm wheel, in a known procedure.

The dimensions of the parts including the worm wheel, worm screw, andcam and bearing surfaces, are such that with maximum permissibleseparation of bearing surface 12:: and

cam surface 12a, and with minimum permissible separation of end face 20xof the worm screw from cam surface 20b, the worm wheel will still applyforce in the direction of the arrow in FIG. 2, against the worm screw,and will have caused movement of the threaded portion of the worm screwnearly into contact with seat 12d, and movement of the unthreaded neck2011 (FIG. 8) of the worm screw nearly into contact with the ridge 12rseparating cam surfaces 12m and Mn on abutment 12c, all as depicted inFIG. 2. When the opposite extreme of permissible dimensional tolerancesexists, that is, when minimum permissible separation of surface 12x fromcam surface l2n exists concurrently with maximum permissible separationof end face 20x from cam surface 20b, as may occur due to manufacturingoperations and variations, the worm wheel merely becomes slightlydeformed at its area of engagement with the worm screw and exerts asomewhat greater lateral force on the worm screw in the direction of thearrow in FIG. 2. In all cases, the threaded portion of the worm screw ispartially within the confines of the arcuate seat 12d, and is precludedfrom significant upward displacement by the overhanging ledge of theelongate abutment 12c, and end face 20x of the worm screw is firmly heldin contact with bearing 12x. Thus the worm screw is positively retainedin place with the surfaces 20x and 12x constantly urged into contact bythe axial component of the force acting on cam surface 20b, andpreventing axial translation of the worm screw. This completeelimination of possibility of undesirable axial translation of the wormscrew is attained without the cost of a separate retaining member andthe attendant expense of usual assembly of such a member into thepotentiometer structure, and irrespective of dimensional variationsamong the components.

The preceding detailed description of the preferred embodiment of theinvention indicates full attainment of the noted objectives. While inthe interest of brevity of disclosure only one specific embodiment ofthe invention has been described, it is evident that changes may be madewithin the true spirit and scope of the invention and will occur tothose skilled in the art in view of the present disclosure. Accordingly,the scope of the invention is defined by the appended claims.

Iclaim:

1. A worm gear adjusted potentiometer comprising:

first means, including a housing comprising a body having a boretherein;

second means, including worm' gearing comprising a worm wheel journaledin said body and a worm screw rotatable in said bore and engaging saidworm wheel for driving the latter incident to rotation of the wormscrew, said worm screw comprising an annular bevel cam surface and saidworm wheel comprising resilient means permitting radial yielding andbeing adapted to resiliently apply force to said worm screw transverselyof the axis of the worm screw;

said first means presenting a bearing surface to absorb axial inwardthrust of said worm screw and further presenting a fixed bevel camsurface engaging said annular bevel cam surface on said worm screw andeffective in conjunction with said force to urge said worm screw axiallyinwardly into engagement with said bearing surface, whereby occurrenceof axial translation of said worm screw in said body is prevented duringadjustment of said potentiometer;

2. A potentiometer as defined in claim I, in which said bearing surfaceis a portion of a smooth interior wall of said body, and in which saidworm screw comprises at the inner end thereof adjacent said annularbevel cam surface a flat circular end face for hearing engagement withsaid bearing surface.

3. A potentiometer as defined in claim 1, in which said fixed bevel camsurface is a surface on an abutment which is an integral portion of saidbody.

4. A potentiometer as defined in claim 3, in which said abutmentcomprises a second fixed bevel cam surface separated from saidfirst-mentioned fixed bevel cam surface by a ridge comprised in saidabutment.

5. A potentiometer as defined in claim 4, in which said worm screwcomprises an outer head portion accessible from the exterior of saidbody and further comprises adjacent the inner end thereof a secondannular bevel cam surface adapted for cooperation with said second fixedbevel cam surface on said body whereby to facilitate assembly of saidworm screw in said body.

6. A potentiometer as defined in claim 5, in which said body is anopen-top boxlike molded structure and comprises an upstanding circularbearing for said worm wheel, said circular bearing being integral withthe floor of said body.

1. A worm gear adjusted potentiometer comprising: first means, includinga housing comprising a body having a bore therein; second means,including worm gearing comprising a worm wheel journaled in said bodyand a worm screw rotatable in said bore and engaging said worm wheel fordriving the latter incident to rotation of the worm screw, said wormscrew comprising an annular bevel cam surface and said worm wheelcomprising resilient means permitting radial yielding and being adaptedto resiliently apply force to said worm screw transversely of the axisof the worm screw; said first means presenting a bearing surface toabsorb axial inward thrust of said worm screw and further presenting afixed bevel cam surface engaging said annular bevel cam surface on saidworm screw and effective in conjunction with said force to urge saidworm screw axially inwardly into engagement with said bearing surface,whereby occurrence of axial translation of said worm screw in said bodyis prevented during adjustment of said potentiometer.
 2. A potentiometeras defined in claim 1, in which said bearing surface is a portion of asmooth interior wall of said body, and in which said worm screwcomprises at the inner end thereof adjacent said annular bevel camsurface a flat circular end face for bearing engagement with saidbearing surface.
 3. A potentiometer as defined in claim 1, in which saidfixed bevel cam surface is a surface on an abutment which is an integralportion of said body.
 4. A potentiometer as defined in claim 3, in whichsaid abutment comprises a second fixed bevel cam surface separated fromsaid first-mentioned fixed bevel cam surface by a ridge comprised insaid abutment.
 5. A potentiometer as defined in claim 4, in which saidworm screw comprises an outer head portion accessible from the exteriorof said body and further comprises adjacent the inner end thereof asecond annular bevel cam surface adapted for cooperation with saidsecond fixed bevel cam surface on said body whereby to facilitateassembly of said worm screw in said body.
 6. A potentiometer as definedin claim 5, in which said body is an open-top boxlike molded structureand comprises an upstanding circular bearing for said worm wheel, saidcircular bearing being integral with the floor of said body.